In a Long Term Evolution-Advanced (LTE-Advanced) system, a CSI-RS is defined. The CSI-RS is a downlink pilot signal, and is an existing signal provided by a sender to a receiver and configured for useful signal channel measurement. CSI feedback may be divided into a periodic feedback mode and a nonperiodic feedback mode. The CSI is formed by a Rank Indicator (RI), a Precoding Matrix Indicator (PMI) and a Channel Quality Indicator (CQI). For nonperiodic CSI feedback, a system working flow is as follows.
1. A base station triggers CSI-RS measurement performed by a user on a certain downlink subframe, and sends a CSI-RS.
2. A terminal obtains a triggering message, measures the CSI-RS on the corresponding subframe, and estimates useful signal quality.
3. The terminal determines CSI according to the useful signal quality.
4. The terminal delays for at least K subframes, and feeds back the CSI on an available uplink subframe (where K refers to a processing time for CSI calculation of the terminal).
5: The base station receives the CSI fed back by the terminal on the corresponding uplink subframe.
A three Dimensions Multiple-Input Multiple-Out-put (3D-MIMO) technology is a topic under discussion of the 3rd Generation Partnership Project Radio Access Network 1 (3GPP RAN1), and 3D-MIMO mainly adopts a two-dimensional array antenna, and may be described with the following parameters (M, N, P, Q), where M represents a number of elements of a column of polarized antennae, N represents a column number of an antenna array, P=2 represents adoption of dual-polarized antennae, MTXRU represents a number of transceiver channels corresponding to the same column of antennae, a transceiver channel may correspond to a plurality of antenna elements, and Q=2N*MTXRU represents a total number of Transceiver Units (TXRUs) included in a TXRU array corresponding to the antenna array. For example, an antenna array corresponding to (M, N, P, MTXRU)=(8,4,2,4) may be represented by FIG. 1 (left), and a TXRU array corresponding to Q=32 may be represented by FIG. 1 (right), TXRU # (mTXRU, nTXRU) representing a certain TXRU.
CSI feedback enhancement is an important topic in 3D-MIMO, and at present, three main solutions are adopted.
A first solution: Full port CSI-RS (all CSI-RS ports)-based CSI feedback, where, in the solution, a number of CSI-RS ports is equal to a number of TXRUs, and they are in a one-to-one mapping relationship, as shown in FIG. 2.
A second solution: Partial port CSI-RS (part of CSI-RS ports)-based CSI feedback, where the solution adopts two sets of independent CSI-RSs, where 2N ports are involved in one set of CSI-RSs, and are mapped to 2N TXRUs in a certain row of a TXRU array one to one, and MTXRU ports are involved in the other set of CSI-RSs, and are mapped to MTXRU TXRUs in a certain column of the TXRU array one to one, as shown in FIG. 3.
A third solution: Beamformed CSI-RS (beamformed CSI-RSs)-based CSI feedback, where, in the solution, the CSI-RSs are beamformed in a vertical dimension, MTXRU TXRUs in the same column are mapped onto B vertical-dimension beams through B groups of different vertical-dimension forming weights, and each vertical-dimension beam corresponds to a set of CSI-RSs (2N ports are involved in each set), so that there are totally B sets of CSI-RSs, as shown in FIG. 4.
The first solution is a method adopted by a current LTE system, and its advantage is that User Equipment (UE) may estimate most comprehensive channel information, that is, the UE may obtain channel information between the UE and all sending antenna ports. However, when the solution is adopted, if a number of ports is relatively large, on one hand, CSI-RS overhead is high, and it is overhead corresponding to different CSI-RS ports. In present LTE, overhead of 8 CSI-RS ports is about 1.5%, overhead of 16 CSI-RS ports is 3%, and overhead of 32 CSI-RS ports and 64 ports is relatively high.
It is widely believed in the industry that: CSI-RS overhead is acceptable when a number of ports is relatively small (for example, smaller than 20), so that the first solution may be adopted; and when the number of the ports is relatively large (for example, larger than 20), CSI-RS overhead of Full port CSI-RS-based CSI feedback is too high, so that the second solution and the third solution are required to be considered.
A system in a related technology supports 1, 2, 4 and 8 CSI-RS ports. CSI-RS pattern designs in FIG. 5a-FIG. 5c may be applied to a frame structure type 1 (Frequency Division Duplex (FDD)) and a frame structure type 2 (Time Division Duplex (TDD)), a Resource Element (RE) configured for CSI-RSs is represented by two letters, the first letter represents a cell index, and the second letter represents antenna ports transmitted on the RE. These patterns adopt a nested structure, that is, 2-port CSI-RS antenna ports are subsets of 4-port and 8-port CSI-RS antenna ports, and such a design is favorable for reducing complexity in implementation. There are totally 40 antenna ports, and it is supported that an intercell frequency multiplexing factor is 5 and each cell adopts 8 CSI-RS antenna ports or the intercell frequency multiplexing factor is 20 and each cell adopts 2 CSI-RS antenna ports. From FIG. 5a-FIG. 5c, it can be seen that a Demodulation Reference Signal (DMRS) (Port 5 is configured to send the DMRS) configured for a Transmission Mode 7 (TM7) in Release 8 (R8) may collide with a CSI-RS, so that it is necessary to avoid scheduling a user of the TM7 in a subframe where the CSI-RS is sent. CSI-RS pattern designs in FIG. 5d-FIG. 5f are only applied to the frame structure type 2 (TDD), mainly for avoiding the DMRS (Port 5) configured for the TM7 in R8 to collide with the CSI-RS. A CSI-RS design in the system of the related technology adopts an orthogonal code division design with a length of 2 in a time domain, so that CSI-RSs of two antenna ports share 2 REs on the same subcarrier.
Configuration of a CSI-RS of up to 8 ports is supported only in the related technology, and how to implement configuration of a 16-port CSI-RS and perform CSI feedback on the basis of the 16-port CSI-RS becomes one of technical problems urgent to be solved.